I've heard conflicting advice on this, from 6 months prior to graduation all the way to "wait until after you've graduated so you can start immediately". On the one hand you don't want to give yourself too little time to interview, but on the other I understand that employers may want you to start immediately to fill a need. Is there anything rude/impolite/weird about applying for a job knowing that you won't be able to accept an offer for several months? Do companies routinely wait on Ph.D.'s to finish?

This may well vary by geography. Employers may be conditioned by usual notice periods, even if these don't apply to new grads. In the US, two weeks' notice is common, so employers may not be used to waiting for months. In contrast, the typical notice period in Germany is three months or even more, so employers will routinely hire someone today who hands in his resignation tomorrow but can only start at the new place three months later. This may also have an impact on expectations for recent graduates. Anyway, you may get (even) better answers at Workplace.
– Stephan KolassaDec 3 '15 at 15:58

@StephanKolassa Good point about this being region-specific, and excellent suggestion for checking out workplace.stackexchange. Quick search there and I came up with this post.
– Sumedh JoshiDec 3 '15 at 16:00

Most positions that hire PhDs are familiar with the nuances of hiring someone who hasn't yet graduated. I currently work in industry, and we've had a number of people who've had to take a few days vacation to defend their thesis. You should mention during the interview process where you currently stand in your PhD work so the employer knows what to expect, but from my experience it shouldn't be too much of a problem.

Do note that some positions require the PhD of their employees, so you would not be eligible for those types of positions until you graduate. I would still recommend interviewing anyways so you're not starting from scratch... you can just contact them and continue when you actually receive the degree.

What about applying for a position that requires a Ph.D. with the understanding that you wouldn't be able to start until after you graduate. Is your company generally fine with waiting a few (3-6) months for students to finish before starting?
– Sumedh JoshiDec 3 '15 at 15:10

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@SumedhJoshi Again, from my experience - you'll have a mixed bag there. In my current position, 3-6 months would not work. For some companies, that's fine. It depends on the nature of the business and the urgency they have in bringing in new hires. No straightforward answer there.
– eykanal♦Dec 3 '15 at 15:13

Some jobs requiring PhDs will happily give you a conditional job offer, conditional on being awarded the degree (in the same way they'll happily make offers "subject to references", or background checks, or whatever). Others will be looking for someone who already has the degree at the time of interview. Your recommendation of course holds in both cases, since applying is a good way to find out which is which ;-)
– Steve JessopDec 4 '15 at 0:19

I have been involved in hiring several Ph.D.'s (math, computer science, operations research) in the finance industry in the US. (before returning to academia to get my own math Ph.D.). In the US, the answer to your question is that the correct time to apply will depend on the type of industry job you are looking for. My experience is that for jobs at larger companies which routinely hire Ph.D.'s you should start applying in the late fall before you anticipate graduating. These companies generally have a well-developed recruitment process and are recruiting with the anticipation of bringing someone on several months down the road. They may even plan to have you start with other recent Ph.D.'s and have some more or less formal training period. (My experience is in finance where this is the norm, other industries may differ.)

For jobs at smaller companies or start-ups, I would recommend waiting. Typically these companies are looking to hire someone who can start as soon as possible (six months is an eternity for a small company or start-up).

So I would recommend starting applications and attending recruitment events for larger companies in your field right now. As the process evolves and the time until you could reasonably starts decreases, you should then expand the range of places you are talking to.

@SumedhJoshi Ha, no you're ok (at least in my experience). If it were February you would be behind. For job applications in industry there are not generally formal deadlines though. They want to hire good people whenever they are available. But, just as a rule of thumb 6 months out is a good amount of time for the recruitment process to play out.
– BambooDec 3 '15 at 15:16

This may also vary along the size of the company and the plans that company has.

One scenario:

If a company wants someone just to complete a task, them maybe if you say on the interview that you will be available after three - six months then the answer will be something like that: We will contact you after this period, and believe me they never call you back. So, a good policy is to make an proposal for part time job untill you finish you Ph.D. (I know this is not easy in all times).

One other scenario:

If you apply to a big company with a long business plan then you can discuss with them when you are able to get started.

My suggestion is to start looking for a job two - three months before you finish your Ph.D.